The use of Diagnostic Imaging for Identifying Abnormal Gas Accumulations in Cetaceans and Pinnipeds

Illustrated cross-sectional computed tomography of the cetacean abdomino-pelvic organs

BACKGROUND

Computed tomography (CT) is a non-invasive diagnostic imaging modality which can be used to study the anatomy and morphology of live or deceased animals in-situ. In cetaceans, existing CT anatomy studies mostly focused on the head and thoracic regions. Using postmortem CT (PMCT) scans of Indo-Pacific finless porpoises (Neophocaena phocaenoides), this study describes the cross-sectional imaging anatomy of the cetacean abdomino-pelvic organs for the first time.

METHODS

PMCT scans of finless porpoises stranded in Hong Kong waters were reviewed, of which two freshly dead cases, one male and one female, were selected for illustration. In addition, a contrast-enhanced PMCT scan was performed on the female subject as a trial for a PMCT-angiography study (PMCTA) in cetaceans. A total of 18 axial PMCT images were acquired at selected vertebral levels in the abdomen and supplemented with a series of corresponding labeled anatomical diagrams.

RESULTS

By applying different image rendering techniques, most osseous and soft tissue structures in the finless porpoise abdomen were successfully depicted and annotated on PMCT, including the male and female reproductive organs in the pelvic region. The application of contrast medium in PMCT created artificial radiodensity differences which improved the ability to visualize and differentiate soft organs and vasculature. The merits and limitations of CT compared to other imaging modalities, as well as the future directions of PMCT in stranding investigation, were discussed.

CONCLUSIONS

The findings from this study significantly enhance the applications of CT in cetaceans by assisting researchers and veterinarians in the interpretation of cetacean abdomino-pelvic CT for morphological and pathological assessment during clinical or postmortem examination.

A Baseline Model For Estimating the Risk of Gas Embolism in Sea Turtles During Routine Dives

Sea turtles, like other air-breathing diving vertebrates, commonly experience significant gas embolism (GE) when incidentally caught at depth in fishing gear and brought to the surface. To better understand why sea turtles develop GE, we built a mathematical model to estimate partial pressures of N2 (PN2), O2 (PO2), and CO2 (PCO2) in the major body-compartments of diving loggerheads (Caretta caretta), leatherbacks (Dermochelys coriacea), and green turtles (Chelonia mydas). This model was adapted from a published model for estimating gas dynamics in marine mammals and penguins. To parameterize the sea turtle model, we used values gleaned from previously published literature and 22 necropsies. Next, we applied this model to data collected from free-roaming individuals of the three study species. Finally, we varied body-condition and cardiac output within the model to see how these factors affected the risk of GE. Our model suggests that cardiac output likely plays a significant role in the modulation of GE, especially in the deeper diving leatherback turtles. This baseline model also indicates that even during routine diving behavior, sea turtles are at high risk of GE. This likely means that turtles have additional behavioral, anatomical, and/or physiologic adaptions that serve to reduce the probability of GE but were not incorporated in this model. Identifying these adaptations and incorporating them into future iterations of this model will further reveal the factors driving GE in sea turtles.

Causes of cetacean stranding and death on the Catalonian coast (western Mediterranean Sea), 2012-2019

The causes of cetacean stranding and death along the Catalan coast between 2012 and 2019 were systematically investigated. Necropsies and detailed pathological investigations were performed on 89 well-preserved stranded cetaceans, including 72 striped dolphins Stenella coeruleoalba, 9 Risso's dolphins Grampus griseus, 5 bottlenose dolphins Tursiops truncatus, 1 common dolphin Delphinus delphis, 1 Cuvier's beaked whale Ziphius cavirostris and 1 fin whale Balaenoptera physalus. The cause of death was determined for 89.9% of the stranded cetaceans. Fisheries interaction was the most frequent cause of death in striped dolphins (27.8%) and bottlenose dolphins (60%). Cetacean morbillivirus (CeMV) was detected on the Catalan coast from 2016 to 2017, causing systemic disease and death in 8 of the 72 (11.1%) striped dolphins. Chronic CeMV infection of the central nervous system was observed from 2018-2019 in a further 5 striped dolphins. Thus, acute and chronic CeMV disease caused mortality in 18% of striped dolphins and 14.6% of all 89 cetaceans. Brucella ceti was isolated in 6 striped dolphins and 1 bottlenose dolphin with typical brucellosis lesions and in 1 striped dolphin with systemic CeMV. Sinusitis due to severe infestation by the nematode parasite Crassicauda grampicola caused the death of 4 out of 6 adult Risso's dolphins. Maternal separation, in some cases complicated with septicemia, was a frequent cause of death in 13 of 14 calves. Other less common causes of death were encephalomalacia of unknown origin, septicemia, peritonitis due to gastric perforation by parasites and hepatitis caused by Sarcocystis spp.

Fatty acid composition and N2 solubility in triacylglycerol-rich adipose tissue: the likely importance of intact molecular structure

Diving tetrapods (sea turtles, seabirds and marine mammals) are a biologically diverse group, yet all are under similar constraints: oxygen limitation and increased hydrostatic pressure at depth. Adipose tissue is important in the context of diving because nitrogen gas (N₂) is five times more soluble in fat than in blood, creating a potential N₂ sink in diving animals. Previous research demonstrates that unusual lipid composition [waxes and short-chained fatty acids (FA)] in adipose tissue of some whales leads to increased N₂ solubility. We evaluated the N₂ solubility of adipose tissue from 12 species of diving tetrapods lacking these unusual lipids to explore whether solubility in this tissue can be linked to lipid structure. Across all taxonomic groups, the same eight FA accounted for 70-80% of the entire lipid profile; almost all adipose tissues were dominated by monounsaturated FA (40.2-67.4 mol%). However, even with consistent FA profiles, there was considerable variability in N₂ solubility, ranging from 0.051±0.003 to 0.073±0.004 ml N₂ ml^-1 oil. Interestingly, differences in N₂ solubility could not be attributed to taxonomic group (P=0.06) or FA composition (P>0.10). These results lead to two main conclusions: (1) in triacylglycerol-only adipose tissues, the FA pool itself may not have a strong influence on N₂ solubility; and (2) samples with similar FA profiles can have different N₂ solubility values, suggesting that 3D arrangement of individual FA within a triacylglycerol molecule may have important roles in determining N₂ solubility.

Pulmonary ventilation-perfusion mismatch: a novel hypothesis for how diving vertebrates may avoid the bends

Hydrostatic lung compression in diving marine mammals, with collapsing alveoli blocking gas exchange at depth, has been the main theoretical basis for limiting N2 uptake and avoiding gas emboli (GE) as they ascend. However, studies of beached and bycaught cetaceans and sea turtles imply that air-breathing marine vertebrates may, under unusual circumstances, develop GE that result in decompression sickness (DCS) symptoms. Theoretical modelling of tissue and blood gas dynamics of breath-hold divers suggests that changes in perfusion and blood flow distribution may also play a significant role. The results from the modelling work suggest that our current understanding of diving physiology in many species is poor, as the models predict blood and tissue N2 levels that would result in severe DCS symptoms (chokes, paralysis and death) in a large fraction of natural dive profiles. In this review, we combine published results from marine mammals and turtles to propose alternative mechanisms for how marine vertebrates control gas exchange in the lung, through management of the pulmonary distribution of alveolar ventilation ([Formula: see text]) and cardiac output/lung perfusion ([Formula: see text]), varying the level of [Formula: see text] in different regions of the lung. Man-made disturbances, causing stress, could alter the [Formula: see text] mismatch level in the lung, resulting in an abnormally elevated uptake of N2, increasing the risk for GE. Our hypothesis provides avenues for new areas of research, offers an explanation for how sonar exposure may alter physiology causing GE and provides a new mechanism for how air-breathing marine vertebrates usually avoid the diving-related problems observed in human divers.